Here is my best answer at this point.
It will depend on the music you are trying to stream. If it is portected music (bought through the apple store pre-drm-free), then you probably won't be able to do this.
Basically create a webpage and embed the songs in it. Then use the browser on the Wii to play them that way. Just turn on your Webserver on your mac, and place the webpage and songs in the /Sites folder under your Home area...you then reach this by going to
http://<the mac's ip>/~<your mac account name>
Pretty sure that at this point that's about the only way to do it.
hope it helps,
Brian